1. Field of the Invention
This invention relates to novel composite oxides of A-site defect type perovskite structure, and more particularly to a novel composite oxide of A-site defect type perovskite structure suitable for use in catalysts for the purification of an exhaust gas discharged from an internal combustion engine of automobiles or the like, catalysts for the combustion of natural gas or the like, catalysts for the absorption and purification of harmful substance such as nitrogen oxides or the like, and electrode materials for solid electrolyte such as zirconia, ceria or the like used in an oxygen sensor or a fuel cell.
2. Description of the Related Art
As the catalyst for the purification of the exhaust gas discharged from the internal combustion engine of the automobile, there have mainly been used ones obtained by carrying noble metals such as platinum, rhodium, palladium and the like on a carrier substrate such as alumina or the like. Particularly, rhodium was used as an essential component for purifying NOx, but is an expensive catalytic material because it is lacking in natural resources.
Recently, levels in quality and quantity required for the purification of the exhaust gas are rising with an increase of consciousness for environments on worldwide scale. Under such situations, the demand of noble metals is extended and hence the shortage and the further rise of cost accompanied therewith are apprehended.
Under the above circumstances, it is attempted to apply palladium, which is relatively rich in natural resources and cheap in the cost, to a catalyst for the purification of the exhaust gas.
In general, it is known that when palladium is used as a catalytic component, reduction of palladium oxide is generated at a high temperature region and hence the NOx purification performance as a palladium catalyst is not sufficiently obtained. In this connection, it has been confirmed that it is effective to select a composite oxide of perovskite structure represented by a rational formula of ABO.sub.3 and add it as a cocatalyst for improving the NOx purification performance of palladium (JP-A-7-136,518 opened May 30, 1995).
However, when such a perovskite type oxide is used as a catalyst or a cocatalyst and carried on alumina as a carrier substrate, if it is used at a high temperature region for a long period of time, the perovskite type oxide reacts with alumina to cause the decrease of specific surface area and the degradation of catalytic performances in the perovskite type oxide.
On the other hand, even if the perovskite type oxide is highly dispersed onto the surface of zirconia substrate to improve the catalytic activity, the degradation is less as compared with the case of carrying on the alumina, but solid phase reaction between perovskite type oxide and zirconia is created at the high temperature region and hence the satisfactory dispersion effect may not be obtained (N. Mizuno et al., J. Am. Chem. Soc., vol. 114, 1992, page 7151).
Particularly, a perovskite type oxide composed mainly of Co is decomposed at a higher temperature in a reducing atmosphere. For this end, it is attempted to include strontium into the perovskite type oxide for improving the catalytic activity through valency control. In this case, it is pointed out that the deactivation is caused by reacting strontium with palladium as a catalytic component at a high temperature (Tanaka and Takahashi, Journal of the Society of Automotive Engineers of Japan, vol. 47, 1993, p51).
Therefore, in order to suppress the degradation of properties at high temperature region in such a perovskite type oxide catalyst composed mainly of Co to improve the durability thereof, it is attempted to simultaneously establish the durability and the catalytic activity by using, for example, La.sub.0.9 Ce.sub.0.1 Co.sub.0.4 Fe.sub.0.6 O.sub.3-.delta. in which an amount of Fe effective for the improvement of resistance to reducing performance is increased and strontium is replaced with cerium as a catalytic component, and using (Ce, Zr, Y)O.sub.2 substrate hardly reacting with perovskite type oxide for controlling solid phase reaction (Tanaka and Takahashi, Journal of the Society of Automotive Engineers of Japan, vol. 47, 1993, p5l).
However, when the perovskite type oxide containing a great amount of Fe is used for improving the thermal resistance, it is difficult to sufficiently obtain a catalytic activity from a low temperature region of an exhaust gas and hence the amount of noble metal used is undesirably increased.
Furthermore, if it is intended to select the composition of the substrate and improve the durability without causing the degradation of properties under severer conditions for coping with the buildup of emission control in North America and Europe and the severer buildup of emission control anticipated in future, the existing perovskite type oxides themselves have limit in use environments.
Moreover, since characteristics of constituent element such as strontium, cerium or the like in the perovskite type oxide appear in the catalytic activity, even if it is intended to prepare a catalyst by utilizing the characteristics of such a perovskite type oxide on the catalytic activity, it is required to control the interaction between the constituent element and noble metal catalyst as far as possible for controlling the degradation of the catalytic activity in the noble metal through the constituent element.
Therefore, it is important to design catalyst materials for the improvement of catalytic activity and durability in the perovskite type oxide itself by utilizing the activity of the perovskite type oxide to effectively conduct valency control.
And also, the perovskite type oxide has recently been used as an electrode material for fuel cell, oxygen sensor or the like, for example, an electrode material for oxygen ion conductor such as stabilized zirconia, ceria or the like. This perovskite type oxide is a mixed ion conductor having a small specific resistivity and a large oxygen ion conductivity. Particularly, it is known that the oxide including a 3d transition element such as Co, Mn or the like is cheap in the cost as compared with a platinum electrode and is possible to operate at a low temperature when using as an electrode material for the oxygen sensor (Y. Takeda, R. Kanno, Y. Tomida and O. Yamamoto, J. Electrochem. Soc., vol. 134, No. 11, 1987, p2656).
In general, when an electrode of a metal such as platinum or the like is used as an electrode material for the oxygen sensor, the penetration of oxygen ion into a solid electrolyte occurs only at a restricted three-phase interface of gas phase, electrode and electrolyte. On the other hand, it has been confirmed that since the penetration of oxygen ion may be caused even at a two-phase interface between electrode and solid electrolyte in an electron-ion mixed conductor electrode, the perovskite type oxide may be a high-performance electrode material operable at a low temperature because a resistance of electrode reaction at an interface between solid electrolyte and perovskite type oxide can be reduced (H. Arai, K. Eguchi and T. Inoue, Proc. of the Symposium on Chemical Sensors (Hawaii), 87-9 (1987), p2247).
Among the perovskite type oxides used as an electrode material, however, a high activity material (La--Co--O system) reacts with zirconia or ceria solid electrolyte during the use for a long time at a high temperature region or under a reducing atmosphere to increase a reaction resistance at an interface between solid electrolyte and perovskite type oxide. As a result, power generation efficiency is decreased in the fuel cell, or the decrease of response velocity and occurrence of abnormal power output are caused in the sensor used at the high temperature region and finally the inoperable state may be caused.
Particularly, the perovskite type oxide can be used as the electrode material for the fuel cell or the oxygen sensor by utilizing the catalytic action and electric conductivity of such an oxide. In this case, however, it is necessary to control the degradation of properties through chemical reaction at the interface between solid electrolyte and electrode material, peeling through thermal stress and the like while maintaining the adhesion property therebetween.
In this connection, there is recently reported only a composite oxide of perovskite structure represented by a rational formula of (La.sub.1-x Sr.sub.x).sub.1-y MnO.sub.3-.delta. usable as an electrode material wherein when y is 0.06, x is 0.08.ltoreq.x.ltoreq.0.30 and when the value of x is 0.11, y is 0.06&lt;y&lt;0.11 (T. Higuchi, M. Miyayama and H. Yanagida, J. Electrochem. Soc., vol. 138, No. 5, 1991, ppl519-1523).